Lab 4-12 – Configuring Inter-VLAN Routing (Router-on-a-Stick)

In this lab you will learn how to configure Inter-VLAN routing, also known as router on a stick by configuring a Switch to trunk to a router with sub-interfaces in each VLAN.

Real World Application & Core Knowledge

The only way to get off a layer two network segment is through a layer three device; commonly referred to as a Default Gateway for host machines. So let’s say for example you have a single switch with 5 different VLAN’s and machines on each VLAN; in which case each VLAN would require its own router to get out of that layer two network to a different layer two network. So does this mean that multiple routers are required or a router with 5 physical ethernet interfaces, one interface in each VLAN?

The answer is easier then you’d initially think. In this case you only need a SINGLE router and that’s it. Think back to the previous lab discussing Dot1q trunk interfaces. 802.1q trunk interfaces carry all VLAN traffic.

A single Router can utilize an 802.1q trunk link to place a sub-interface in each VLAN using a single physical link and technically have interfaces in all VLAN’s.

A Sub-Interface is a logical interface partitioned off from a physical interface. A sub-interface allows you to have multiple interface configurations on a single physical interface. In this lab you’ll use sub-interfaces to match the VLAN’s in the trunk to allow for interfaces in each VLAN and accomplish inter-vlan routing for the hosts as the router has an interface in each layer three network.

In this lab you’ll familiarize yourself with the following new commands;

encapsulation dot1Q # – This command is executed from ethernet sub-interface configuration mode and binds the sub interface(s) to a particular 802.1q tagged vlan.

no ip routing – This command is executed from global configuration and disables the routers ability to be used as a router, effectively turning it into a test client machine like a windows box.

ip default-gateway 1.2.3.4 – This command is executed in global configuration mode to configure a non-routing device to use the specific IP Address as a default-gateway.

show ip interface brief – This command can be executed from user or privileged mode to view the current IP addresses of all interfaces on the device.

Lab Prerequisites

• If you are using GNS3 than load the Stub Area Networking GNS3 topology than start devices; R1, R2, R3 and SW1.
• Establish a console session with devices R1, R2, R3 and SW1 than configure the devices respected hostname(s).
• Create VLAN’s 20 and 30 on SW1 and configure interface Fa0/1 on SW1 as an 802.1q trunk link.
• On SW1 configure interface Fa0/2 to access VLAN20 and Fa0/3 to access VLAN 30.
• Configure the IP address 10.1.20.2/24 on R2′s FastEthernet0/0 interface.
• Configure the IP address 10.1.30.3/24 on R3′s Fastethernet0/0 interface.

Lab Objectives

• Configure a new Sub-Interface on R1 to match the VLAN 20 (Fa0/0.20) and configure the sub-interface to use 802.1q encapsulation and the Dot1q tag of 20. Configure the sub-interface to use the IP address 10.1.20.1/24.
• Configure a new Sub-Interface on R1 to match the VLAN 30 (Fa0/0.30) and configure the sub-interface to use 802.1q encapsulation and the Dot1q tag of 30. Configure the sub-interface to use the IP address 10.1.30.1/24. Verify your sub-interface configuration.
• Disable IP Routing on R2 and R3 and configure the default gateway on R2 and R3 to use R1′s respected Sub-interface as the default gateway.
• Verify that R2 can ping R3′s FastEthernet0/0 interface using R1 as the default-gateway.

Lab Instruction

Step 1. – Configure a new Sub-Interface on R1 to match the VLAN 20 (Fa0/0.20) and configure the sub-interface to use 802.1q encapsulation and the Dot1q tag of 20. Configure the sub-interface to use the IP address 10.1.20.1/24.

To create a new sub-interface you’ll use the interface fa0/0.# command in global configuration mode. To enable the sub-interface to use 802.1q you’ll use the encapsulation dot1q # command whereas # is the dot1q VLAN tag as shown below;

R1 con0 is now available

Press RETURN to get started.

R1>enable
R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#interface fa0/0
R1(config-if)#no shut
R1(config-if)#interface fa0/0.20
R1(config-subif)#encapsulation dot1q 20
R1(config-subif)#ip add 10.1.20.1 255.255.255.0
R1(config-subif)#exit
R1(config)#

Step 2. – Configure a new Sub-Interface on R1 to match the VLAN 30 (Fa0/0.30) and configure the sub-interface to use 802.1q encapsulation and the Dot1q tag of 30. Configure the sub-interface to use the IP address 10.1.30.1/24. Verify your sub-interface configuration.

R1(config)#interface fa0/0.30
R1(config-subif)#encapsulation dot1q 30
R1(config-subif)#ip add 10.1.30.1 255.255.255.0
R1(config-subif)#end
R1#sh run interface fa0/0.20
Building configuration...

Current configuration : 96 bytes
!
interface FastEthernet0/0.20
 encapsulation dot1Q 20
 ip address 10.1.20.1 255.255.255.0
end

R1#sh run interface fa0/0.30
Building configuration...

Current configuration : 96 bytes
!
interface FastEthernet0/0.30
 encapsulation dot1Q 30
 ip address 10.1.30.1 255.255.255.0
end

R1#show ip interface brief | inc FastEthernet0/0
Interface               IP-Address    OK? Method   Status     Protocol
FastEthernet0/0         unassigned    YES unset    up         up
FastEthernet0/0.20      10.1.20.1     YES manual   up         up
FastEthernet0/0.30      10.1.30.1     YES manual   up         up
R1#

Step 3. – Disable IP Routing on R2 and R3 and configure the default gateway on R2 and R3 to use R1’s respected Sub-interface as the default gateway.

To disable IP Routing on R2 and R3 use the no ip routing command in global configuration. To specify a default gateway use the ip default-gateway x.x.x.x as shown below;

R2 con0 is now available

Press RETURN to get started.

R2>enable
R2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#no ip routing
R2(config)#ip default-gateway 10.1.20.1
R2(config)#end
R2#

R3 con0 is now available

Press RETURN to get started.

R3>enable
R3#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R3(config)#no ip routing
R3(config)#ip default-gateway 10.1.30.1
R3(config)#end
R3#

Step 4. – Verify that R2 can ping R3’s FastEthernet0/0 interface using R1 as the default-gateway as shown below;

R2#ping 10.1.30.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.30.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/23/52 ms
R2#